1. Field of the Invention
The invention relates to permeable membrane systems. More particularly, it relates to the use of such systems under variable demand conditions.
2. Description of the Prior Art
Permeable membrane processes and systems are known in the art and are being employed or considered for a wide variety of gas and liquid separations. In such operations, a feed stream is brought into contact with the surface of a membrane, and the more readily permeable component of the feed stream is recovered as a permeate stream. The less readily permeable component is withdrawn from the membrane system as a non-permeate, or retentate, stream.
The inherent simplicity of such fluid separation operations provides an incentive in the art to expand use of membranes in practical commercial operations. It is necessary, of course, that the selectivity and permeability characteristics of a membrane system be compatible with the overall production requirements of a given application. It is also necessary that the efficiency of membrane systems be continually improved in order to enhance the feasibility of employing membrane systems to advantage under the operating conditions encountered in the art.
Significant factors in the design and overall efficiency of membrane systems are the total membrane surface area required for a given separation and the partial pressure difference across the membrane that is required to obtain a desired product quantity and quality, i.e. a desired permeability and selectivity or separation factor. The design of practical membrane systems requires optimization of the trade-offs between membrane surface area and said partial pressure differences. Thus, the greater the partial pressure difference, or driving force, across the membrane, the less is the membrane surface area required for a given separation. This necessitates the use of more expensive pumping equipment and higher pump operating costs, but enables membrane equipment costs to be kept relatively low. If, on the other hand, a lower driving force is employed, more membrane surface area is required, and the relative costs of the various aspects of the overall system and operation would change accordingly.
Membrane systems are usually designed and optimized for full capacity, steady constant flow conditions, i.e. design conditions, that are not always fully utilized in practice. Under operating conditions other than the design conditions, different combinations of optimum operating conditions will prevail with respect to membrane area versus partial pressure differences. Fluid separation applications for which membrane systems are desirable typically do not run under steady flow conditions. The demand from the membrane system will often vary in terms of product quantity and/or quality. For example, product demand for nitrogen gas from an air separation membrane system can vary significantly in a 24 hour period in terms of nitrogen flow rate and/or purity required.
Membrane systems typically operate in one of three modes during off demand, or so-called turndown conditions. In one approach, there is no turndown to accommodate decreases in product demand. In this case, the feed flow and partial pressure differences remain constant. The product quality will, as a result, increase to above design level, while the power requirements will remain at the full design level. This approach is thus disadvantaged in that no power reduction is realized, and the product is obtained at greater than required quality levels.
In another approach, the membrane surface utilized for separation is varied. Under reduced demand conditions, a portion of the membrane area is shutdown. This reduces the amount of the feed stream that is processed in order to satisfy the reduced demand. The disadvantage of this approach is the inefficiency associated with the lack of use of available membrane surface area. This factor assures that the trade-off between membrane surface area and partial pressure difference is not optimized for off-design turndown conditions.
In a third approach to reduced demand conditions, the membrane system is operated at design conditions when in operation, and a surge tank is used to handle variable demand requirements. When the surge tank is full, the membrane system is either unloaded, i.e., the feed pumps are idled, or shutdown to save energy. This approach also has the disadvantage of not fully utilizing installed membrane area since this membrane area is not utilized at all during periods of plant idle or shutdown. Such start/stop operation also has the disadvantage of increased wear on associated equipment.
There is a genuine need in the art, therefore, for a method of fully and efficiently utilizing the separation capability of the installed membrane area under turndown conditions. Such a method would enable an installed membrane system to operate at an optimum efficiency at all times, and would provide more reliable operation than the start/stop mode referred to above.
It is an object of the invention, therefore, to provide an improved method for operating a permeable membrane system under turndown conditions.
It is another object of the invention to provide a method for enabling the full separation capacity of an installed membrane system to be efficiently employed under reduced product demand conditions.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.